Process of making a ferromagnetic core having a predetermined permeability



May 30, 1961 F. G. BROCKMAN 2,985,939

PROCESS OF MAKING A FERROMAGNETIC CORE HAVING A PREDETERMINEDPERMEABILITY Original Filed July 10, 1952 2 Sheets-Sheet 1 TEMPE/Q471/85 INVENTOR FRANK G, EEOC/(MAN fix/2W AE/VT TEMPE/2.4 rules M y 1961F. G. BROCKMAN 2,985,939

PROCESS OF MAKING A FERROMAGNETIC CORE HAVING A PREDETERMINEDPERMEABILITY Original Filed July 10, 1952 2 Sheets-Sheet 2 F'oQM IPEQQITE LAYEIZ FTQQM 2ND Fazzrre LAYEB P2555 Bo-rH LAYEQS A'r H5500lbg./|n

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AGE/VT United States Patent PROCESS OF MAKING A FERROMAGNETIC COREHAVING A PREDETERMINED PERMEABILITY Frank G. Brockman, Dobbs Ferry, N.assignor to Philips Laboratories, Inc, Irvingtou-on-Hudson, N.Y.

1 Claim. (Cl. 25-157) My invention relates to a magnetic core in whichvariations of permeability with temperature can be controlled. Inparticular, the invention relates to magnetic cores comprisingessentially one or more ferrites.

Theferrites are compositions of one or more oxides of certain bivalentmetals and ferric oxide which have been sintered to produce a materialhaving very high values of initial permeability. These materials havebeen fully described in US. Patents #2,452,529, #2,452,- 530,#2,452,531, #2,551,7ll, 2,579,978 and in a monograph by J. -L. Snoek,New Developments in Ferromagnetic Materials, 1947. The materialsdisclosed in those patents have a high initial permeability, i.e.,several hundred or more, and a low loss factor, which makes themparticularly applicable at high frequencies in induction coils for, forexample, permeability tuning.

An important property required of magnetic cores for such inductioncoils is stability-the inductance should vary as little as possible withtemperature. In general, it is exceedingly difiicult, if not practicallyimpossible, to obtain a ferrite core having substantially constantpermeability over a broad range of temperatures.

There are other applications for ferrite cores in which it might bedesirable to obtain a core which exhibits rapidchanges in permeabilitywith changes in temperature. Such cores may, for example, be utilized intemperature measuring devices or as a temperature compensating elementto compensate for changes in inductance of some other circuit element.In such cases, the composition of the ferrite material alone does notprovide suflicient flexibility of control of thepermeability-temperature characteristic of the core.

The present invention is directed to a novel technique for obtainingferrite cores having any desired permeability-temperaturecharacteristic. In particular, I have found that it is possible tocontrol the permeability versus temperature curve of a ferrite core byforming a laminar body of two or more ferrite materials having differentpermeability-temperature characteristics which will result in a bodyhaving a new characteristic. This technique is particularly advantageousbecause the new permeabilitytemperature characteristic of the laminarbody can be approximately predicted from the permeability-temperaturecharacteristics of each of the ferrites used so that it is possible fromthe characteristics of the ferrites alone to determine, in advance, whatferrites and the amounts thereof which are necessary to produce therequired characteristic.

In accordance with my invention I am able to determine empirically uponthe basis of the following formula the relative proportions of the twoor more ferromagnetic ferrites constituting separate laminae in thecore. I have found that the resultant pemeability ,u, of the coreaccording to my invention is related to the cross-sectional areas A andA respectively of the laminar sections constituted of differentferromagnetic ferrites having permeabilities a and 11. respectively bythe relationship:

ice

A being the total cross-sectional area of the core perpendicular to thepath of the flux.

A ferrite core in accordance with the invention can be prepared byassembling two or more cores of the same geometrical shape each of whichhas a desired temperature-permeability characteristic chosen tocompensate for temperature variations in permeability of other portionsof the core. The cores are assembled so that the path of flux isparallel to the abutting faces of each of the cores and the cores joinedtogether either mechanically or by firing at a temperature at which thecores fuse without diffusion of one core material into another.

A preferred arrangement is a laminae core consisting of two sections;one section having a low Curie point (below about C. and above roomtemperature) and the other section having a higher Curie point (betweenabout 110 C. and about 250 C.), which arrangement has the advantage thatrelatively flat temperature-permeability characteristics are obtained.

The core is preferably prepared by placing in a mold a first ferritematerial in powder form and levelling its surface and placing thereovera second ferrite in powder form. The core is then pressed and sinteredto produce a laminar core of at least two different ferrites. 'Infiring, there may be some diffusion of one ferrite into another. Thismay be minimized by a careful choice of the sintering temperature, f.i.,by maintaining it at a sufficiently low value consistent with producinga core having mechanical stability or by limiting the sintering time.

The invention will be described with reference to accompanying drawingin which:

'Fig. 1 shows, in cross-section, ing to the invention;

Fig. 2 shows curves of permeability plotted against temperature of twoferrite materials used in the core and a like curve for a composite coremade up of laminae of the two materials;

Fig. 3 shows a curve of permeability plotted against temperature of twoferrite materials and a like curve for a composite core made up oflaminae of those materials;

Fig. 4 shows another embodiment of a core according to the invention;

Fig. 5 shows curves of permeability plotted against temperature of twoferrite materials and a like curve for a composite core formed bymechanically joining two cores of the two ferrites.

The core 1 shown in Fig. 1 is composed of two laminae 2, 32, each ofwhich consists of a particular ferrite having a temperature dependenceof permeability illustrated by curves 10 and 12 in Fig. 2. Thetemperature permeability characteristic of the composite core is shownby the curve designated 15 in Fig. 2.

Each of the laminae constituting the core consists of a nickel-zincferrite of the type described in the abovenoted patents. While I havechosen to illustrate by invention with those ferrites, I wish it to beunderstood that I could have illustrated the invention with other typesof ferrites described in those patents such as copper-zinc ferrite,copper-cadmium ferrite, magnesium-zinc ferrite, manganese-Zinc ferriteor any combination of those ferrites.

The first ferrite was prepared by mixing 15 mol. percent of NiO, 35 mol.percent of ZnO and 50 mol. percent a laminar core accordof Fe Osintering the mixture as described in the patcuts and pulverizing theferrite mass thereby produced. The second ferrite was prepared by mixing20 mol. percent of NiO, 30 mol. percent of ZnO and 50 mol. percent of FeO sintering and pulverizing the sintered mass in exactly the same manneras in the case of the first ferrite.

The first ferrite powder was placed in a ring-shaped mold, its surfacecarefully levelled off and the second ferrite powder placed over thefirst ferrite in a ratio of nine parts of the second ferrite to one partof the first ferrite powder in the mold, the powders pressed at apressure of about 15,100 lbs/in and sintered at 1250" C. for one hour.The resulting core had a laminar structure and a temperaturepermeability characteristic as shown in Fig. 2 by the solid linedesignated 15.

A similar core was prepared by varying the proportions of the twoferrites. Fig. 3 shows the temperature-permeability curves of theindividual ferrites as well as the temperature-permeability curve of thecomposite core. The composite core consists of two laminae, eachrespectively, consisting of the ferrites described above but in equalproportions of the two ferrites. Curve 16 is thetemperature-permeability characteristics of the com posite core.

An alternative embodiment of my invention is shown in Fig. 4. Instead offusing the two ferrites by sintering, ring-shaped sections 6 and 7 eachof one of the ferrites described above can be prepared in the customarymanner and placed one on top of the other and held together by amechanical clamp 8. Furthermore, instead of the mechanical clamp, thecores can be cemented together with a nonmagnetic binder such as anethoxyline resin which is aliphatic-aromatic chain molecules having attheir ends a reactive ethylene oxide group.

Fig. 5 shows the temperature-permeability characteristics of a ferritecore formed by mechanically clamping together two independently madecores as shown in Fig. 4. In this case, the upper core 6 was prepared bymixing 17 mol. percent of NiO, 33 mol. percent of ZnO and 50 mol.percent of Fe O sintering the mixture as described in the patents andthoroughly pulverizing the ferrite mass thereby produced. Thereafter,the ferrite powders were placed in a ring-shaped mold and pressed andsintered in the same manner as that described in connection with thecomposite core shown in Fig. 1. The lower core 7 was prepared by mixing20 mol. percent of NiO, 30 mol. percent of ZnO and 50 mol. percent of FeO and thereafter processing in the same manner as the upper core 6. Thetemperaturepermeability characteristic of the upper core 6 is shown bythe dashed curve 20 in Fig. 5 and the characteristic of the lower core 7by the solid curve 21 in Fig. 5. The resultant permeabilitycharacteristic, as shown in Fig. 5 by the dash-dot curve designated 22,results from mechanically clamping cores 6, 7 of equal thicknesstogether by a non-magnetic clamp 8 (Fig. 4).

While I have thus described my invention with reference to specificexamples and application, I do not wish to be limited to those examplessince other modification will be readily apparent to those skilled inthe art without departing from the spirit and scope of my invention asdefined in the appended claim.

What I claim is:

In the process of manufacturing a ferromagnetic core having a giventemperature coetlicient of permeability, the steps of forming a firstlayer of a nickel-zinc ferrite powder having a composition correspondingto about 15 mol percent of NiO, about 35 mol percent of ZnO, and aboutmol percent of FeO forming over said first layer a second layer ofnickel-zinc ferrite powder having a composition corresponding to about20 mol percent of NiO, about 35 mol percent of ZnO, and about 50 molpercent of Fe O compressing said powders, and sintering said powders ata temperature of about 1250 C. for about one hour to form a core havinga laminar structure.

References Cited in the file of this patent UNITED STATES PATENTS1,141,134 Metzner June 1, 1915 1,448,684 Beecher et al. Mar. 13, 19232,158,132 Legg May 16, 1939 2,218,196 Hagar Oct. 15, 1940 2,241,441Bandur May 13, 1941 2,267,954 Schumacher Dec. 30, 1941 2,551,711 Snoeket al. May 8, 1951 2,648,804 Steigerwalt et al. Aug. 11, 1953 2,830,320Geppert Apr. 15, 1958 OTHER REFERENCES Harvey et al.: FerromagneticSpinels for Radio Frequencies, RCA Review, Sept. 1950, vol. XI, No. 3pages 344-362.

UNITED STATES PATENT OFFICE CERTIFICATION OF CORRECTION Patent No.2,985,939 I May 30, 1961 Frank G. Brockman It is hereby certified thaterror appears in the above numbered patent requiring correction and thatthe said Letters Patent should read as corrected below.

In the grant, lines 2 and 3, for "assignor to Philip's Laboratories,Inc., of Irvington-on-I-Iudson, New York," read assignor, by mesneassignments, to North American Philips Company, Inc. of New York, N. Y.a corporation of Delaware, line 12, for "Philips Laboratories, Inc. itssuccessors" read North American Philips Company, Inc. its successors inthe heading to the printed specification, lines 4 and 5, for "assignorto Philips Laboratories, Inc., Irvington-on- Hudson, N. Y,?, readassignor, by mesne assignments, to North American Philips Company, Inc.,New York, N. Y. a corporation of Delaware column 4, line 19, for "FeOread Fe 0 line 22, for ,"35" read 3O Signed and sealed this 7th day ofNovember 1961.

(SEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD Attesting Officer Commissioner of Patents

